This invention relates in general to wheelchairs and more particularly, to wheelchair suspension systems. Most particularly, the invention relates to resilient wheelchair suspensions that support the wheelchair wheels independently of one another.
Wheelchairs generally include a frame that supports a pair of drive wheels and a pair of front casters. The drive wheels and casters are typically rigidly supported by the wheelchair frame. The drive wheels make contact with the ground and are driven to propel the wheelchair. The drive wheels may be driven manually or powered by an electrical motor. The wheelchair supports a seat assembly comprising a seat and a backrest. The seat assembly is oriented above and between the drive wheels and the front casters to provide stability.
A problem with wheelchairs is that the drive wheels and casters typically fail to maintain contact with the ground as the wheelchair negotiates obstacles or irregular ground surfaces. Various attempts have been made to overcome this problem. One such attempt is set forth in U.S. Pat. No. 4,128,137, issued to Eric Booth. The Booth patent describes a suspension comprising a plate carrying a wheel unit and a bogie system. The bogie system comprises two bogie units. Each bogie unit includes a drive wheel and a caster wheel attached to a frame member. The frame members pivot about a common transverse axis defined by brackets mounted to the plate to maintain contact with the ground. Another attempt to maintain contact between the wheel and the ground is set forth in International Patent Application No. PCT/SE89/00647, which describes a wheelchair chassis having a central, transverse main shaft with two drive wheels, and a front and rear pair of wheels, each carried by individual supporting arms which are swingable in a plane.
Another problem with wheelchairs is that they have a tendency to tip backwards upon initial acceleration, or when ascending up an inclined surface or traversing a curb. In light of this problem, anti-tip caster wheels are often provided rearward of the drive wheels. These anti-tip caster wheels are often mounted on arms rigidly coupled to the support frame and are angled downward to a level just above the ground. As the wheelchair begins to tip backwards, the anti-tip caster wheels engage the ground to prevent further tipping.
While anti-tip mechanisms have successfully prevented rearward tipping of the wheelchair, the rigid coupling of the anti-tip caster wheels to the support frame usually provides a fairly abrupt jolt to the wheelchair occupant as the anti-tip caster wheels engage the ground. U.S. Pat. No. 5,435,404, issued to Paul V. Garin III, describes an anti-tip device comprising a spring-biased arm for supporting a rear caster assembly. The spring-biased arm is provided to absorb shock and provide greater comfort for the wheelchair occupant. In addition to absorbing shock, the spring-biased arm is provided to insure that substantially all the wheels and casters maintain contact with the ground.
Another anti-tip configuration is described in U.S. Pat. No. 5,351,774, issued to James Okamoto. The Okamoto patent describes an anti-tip suspension system comprising a pair of suspension arm assemblies pivotally mounted to opposite sides of a wheelchair frame. The suspension arm assemblies are formed to provide variable rate resistance to rearward tipping. The resistance progressively increases as the anti-tip suspension engages the ground. When the suspension arm assemblies are vertically compressed, the resistance rate of the anti-tip suspension increases non-linearly, further resisting rearward tipping.
A resilient device having deformable cushions is described in U.S. Pat. No. 2,729,442, issued to Hermann J. Neidhart. The device includes an outer member and an inner member, each of which are generally square in cross-section. Pockets are formed between the outer and inner members for receiving elastic cushioning elements. The outer and inner members may be of any desired length and are substantially concentric with each other and mounted for relative coaxial rotation. The cushioning elements are in the form of round rods and made of rubber. The cushioning elements are radially compressed to fit in the pockets. When a load is applied to the device in such a manner so as to rotate the inner member relative to the outer member, the cushioning elements are compressed between and outer and inner members. When the load is released, the device has rebound characteristics that cause relative rotation of the outer and inner members in a reverse direction. A lever arm may be rigidly connected to the inner member for applying the load, and the outer member will be held against rotation. Similar devices are described in U.S. Pat. Nos. 5,591,083, issued to Gerhard Kirschey, 3,783,639, issued to Derek J. Goodman et al., 3,482,464, issued to Heinrich Reich et al., 3,436,069, issued to Curtiss W. Henschen, and 2,712,742, issued to Neidhart.
Torsion axles similar to the above described device are described in U.S. Pat. Nos. 5,411,287 and 5,277,450, issued to Curtiss W. Henschen, 5,411,286, issued to Jerry W. Pittman, 4,966,386, issued to Anton Werdich, 4,655,467, issued to James A. Kitzmiller et al., 3,687,479, issued to Kurt Kober, and 3,353,840, issued to Richard R. Love. Suspension systems similar to the above described device are described U.S. Pat. Nos. 3,779,576, issued to George D. Malcolm, and 3,601,424, issued to Barrie J. Badland.